1. Field of the Invention
The present invention relates to a management system of a secondary battery module. More particularly, the present invention relates to a secondary battery module management system for precisely calculating a State of Charge (SOC), and a driving method thereof.
2. Description of the Related Art
Recently, high power secondary batteries using non-aqueous electrolytes having a high energy density have been developed, and a large capacity secondary battery is formed by coupling a plurality of high output secondary batteries in series so that the secondary battery can operate an apparatus requiring high power, such as a motor of an electric vehicle.
As described, one large capacity secondary battery (hereinafter, referred to as a battery module for convenience of description) includes a plurality of secondary batteries (hereinafter referred to as unit batteries) coupled in series.
In the above battery module, more specifically, in a secondary battery module for a Hybrid Electric Vehicle (HEV), several to tens of unit batteries are alternately recharged and discharged, and therefore it is necessary to control a charge/discharge operation of the battery modules so as to maintain them in an appropriate operational state.
Accordingly, the battery module for the HEV detects voltage, current, and temperature of the unit batteries, estimates the State of Charge (SOC), and performs an SOC control operation so as to increase power consumption efficiency of a vehicle. In this case, an SOC level is required to be controlled for a good balance of a power assist operation for driving a motor during an acceleration of a vehicle and an energy recovery operation (regenerative braking) during a deceleration of the vehicle. Accordingly, the battery module is controlled so that the SOC level is within a range of from 50% to 70%. When the SOC level becomes close to 70% while the battery module is charged, the battery module is controlled so as not to be overcharged. In addition, when the SOC level becomes close to 50% while the battery module is discharged, the battery module is controlled so as not to be over-discharged.
To appropriately perform the SOC control operation, it is necessary to accurately calculate the SOC level of the secondary battery being charged and discharged.
Conventional SOC calculation methods for commercialized batteries are classified according to a reference value for determining the SOC.
The conventional SOC calculation methods include: an AH method which calculates the SOC by using a relationship between the current used (amperes) and time, a voltage estimation method which measures an Open Circuit Voltage (OCV) to calculate the SOC by using a relationship between the measured OCV and the SOC, and a resistance estimation method which calculates the SOC by using a relationship between the SOC and an Internal Resistance drop (IR-drop) of the battery.
However, in the AH method, there is a problem in that the SOC according to a load condition of the battery module may not be displayed and an SOC level error is increased by a charge/discharge current sensing error. Accordingly, it is difficult to use the AH method alone in a Hybrid Electric Vehicle (HEV) since the amount and direction of the currents vary more frequently than in an Electric Vehicle (EV). In the voltage estimation method, there is a problem in that variations occur for various reasons, such as an instant current, temperature, and degradation of the battery.